Effect of water salinity on the hydro-mechanical behaviour of light backfill material

A series of one-dimensional compression tests was conducted to examine and compare the hydro-mechanical behaviour of light backfill (LBF) material, composed of 50% bentonite and 50% sand, in the presence of distilled water and 100 g/l CaCl2 and 200 g/l CaCl2 solutions. In addition, the hydro-mechanical parameters of LBF required for compliance modelling of the deep geologic repository (DGR) emplacement room sealing system were determined. The tests were conducted on 50-mm-diameter by 10-mm-thick LBF samples, using standard lever arm consolidation equipment. Several different loading and wetting paths were examined, including allowing the LBF to swell up to 20% vertical strain on distilled water or solution uptake and constraining the LBF from swelling on distilled water or solution uptake. The samples were loaded in increments following initial distilled water or solution uptake, and then unloaded in increments.The test results show that the compression, swelling and hydraulic behaviour of LBF with 100 g/l or 200 g/l CaCl2 cell reservoir solution is distinctly different than the behaviour of LBF with distilled water in the cell reservoir. All of the test results show significant hysteresis between the loading/compression and unloading/swelling paths. The results of the simulations show that the water activity values in HCB and LBF in the hypothetical emplacement room sealing system are greater than the 0.96 threshold value.The results of individual test loading and unloading increments were used to compute void ratio (e), hydraulic conductivity (k), effective montmorillonite dry density (EMDD), bulk modulus (K) and water activity (aw). The hydraulic conductivity versus EMDD and vertical applied pressure versus EMDD results were compared to results compiled by Dixon et al. (2002a). The bulk modulus versus vertical strain results were used to simulate the interaction between highly compacted bentonite (HCB) (inner material) and LBF (outer material) in a hypothetical emplacement room sealing system, using a two-material axisymmetric linear elastic analytical model.